Fabry-Perot (FP) etalons are used as optical frequency discriminators in devices that monitor and control the optical frequency of optical sources such as semiconductor laser diodes (the terms optical sources and light sources are used interchangeably herein). An attractive feature of FP etalons is the periodic nature of their transmission and reflection (i.e., optical) characteristics. The periodicity or free spectral range (FSR) of the etalon optical characteristics can be matched to characteristics of the optical system in which the etalon is to be employed by selecting an etalon having a specific optical thickness, which is defined by the product of the etalon physical thickness and its refractive index. For example, the FSR of an etalon can be matched to the channel spacing of a wavelength division multiplex (WDM) optical communication system.
Manufacture of etalons with a precisely specified FSR requires tight control of the etalon physical thickness. Typically, the FSR of a FP etalon can be controlled to better than 0.5% for a FSR range of approximately 100 GHz, and a wavelength of approximately 1,550 nanometers (nm). However, the absolute frequencies of the peaks, valleys and flanks of the FP optical characteristic, which are determined by the phase of the interference of light signals within the etalon, are far more difficult to control in manufacture. Unfortunately, it is this phase of optical characteristics which permits a FP etalon to operate as a frequency discriminator. Consequently, control of this characteristic is essential to using FP etalons as frequency discriminators.
Variations in the physical thickness of an etalon will cause the phase of optical characteristic of the etalon to shift. The phase of an FP etalon optical characteristic will vary over a range of 2.pi. radians (360.degree.) for each change in optical thickness equal to 1/2 of one optical wavelength within the etalon. For example, for a wavelength of 1550 nm and a 1 mm thick etalon of refractive index of approximately 1.5, a change of physical thickness of approximately 520 nm causes a 2.pi. change of phase in the etalon optical characteristic. To set the phase characteristic of the etalon at a particular value, i.e., 0.+-..pi./10 radians, the etalon thickness cannot vary more than .+-.26 nm over its entire surface area. Such constrained manufacturing tolerances are far beyond the capabilities that exist today. In other words, it simply is not feasible to manufacture a FP etalon to achieve a preset phase of optical characteristic. Rather, in manufacture, the phases of a batch of etalons are randomly distributed and each etalon must be manually tuned to provide the desired optical characteristic.
In practice, a single optical detector is used in connection with the FP etalon and the desired phase of FP optical characteristic is obtained by mechanically rotating or tuning the position or angle of the etalon relative to the beam of light incident upon it. Rotation of the etalon effectively varies its thickness as seen by the incident light beam. For example, an etalon can be rotated relative to an incident beam while monitoring the optical characteristic of a transmitted or reflected beam until a desired result is achieved. This operation, involving sequential and repetitive tuning and monitoring, is referred to as active alignment and is time-consuming and expensive. Passive alignment, on the other hand, in which an etalon is fixed in place before testing, is considered to be a more cost effective and preferred alternative for setting the etalon's phase of optical characteristic. Unfortunately, due to the manufacturing tolerances in etalon thickness, passive alignment does not provide an acceptable solution when a single detector is used.
There thus exists a need in the art for a method of obtaining a desired phase of optical characteristic of a FP etalon, without specifying physical dimensions of the etalon to tighter tolerances then are currently practical, and without requiring active alignment to tune the etalon.